1. Field of the Invention
This invention relates to a process for producing high-density nickel hydroxide for use as a positive electrode active material in alkali batteries that employ cadmium, zinc and other hydrogen-absorbing alloys at the negative electrode.
2. Description of the Prior Art
With the recent efforts toward adding higher value to portable electronic devices, it has become necessary to use batteries having increased capacity. With alkali batteries, this requirement can be met by enhancing the utilization of the nickel hydroxide powder which is used as the active material in a nickel paste positive plate. The addition of cobalt hydroxide has been proposed as a method of insuring electrical conductivity between nickel hydroxide particles to thereby enhance their utilization. In fact, however, cobalt hydroxide is distributed so unevenly in the paste that it has to be used in an increased amount in order to realize uniform dispersion; but then the loading of nickel hydroxide decreases and the drop in battery's capacity is unavoidable. It is known to add cobalt monoxide for the same purpose but it is generally held that the fabricated battery does not have satisfactory discharge performance on account of the formation of Co.sub.3 O.sub.4 which contributes in no way to improve the utilization of nickel hydroxide particles.
Under the circumstances, it has been proposed that electrical conductivity be insured between nickel hydroxide particles to enhance their utilization by coating the surfaces of the nickel hydroxide particles with cobalt hydroxide in one of the following manners:
1) To obtain a uniform cobalt oxyhydroxide layer within a liquid electrolyte, the cobalt hydroxide is not directly mixed with nickel hydroxide. Instead, the particles of nickel hydroxide are first impregnated in a cobalt salt solution and reacted with sodium hydroxide to form a .beta.-cobalt hydroxide coating layer on the surfaces of the nickel hydroxide particles (see Japanese Patent Public Disclosure No. 152866/1988);
2) the particles of nickel hydroxide are immersed in a cobalt salt solution and reacted with an alkali hydroxide at a controlled pH, thereby forming cobalt hydroxide layer on the surfaces of the nickel hydroxide particles; in order to compensate for the cracks that will develop in the cobalt hydroxide coating layer during the pressing of the electrode plate, elemental Co or a Co compound is added, thereby forming a conductive network in the liquid electrolyte (see Japanese Patent Public Disclosure No. 235303/1995);
3) the particles of nickel hydroxide are immersed in a solution having a cobalt salt dissolved in a mixture of water and ethyl alcohol and the nickel hydroxide is reacted with sodium hydroxide under agitation at a pH of 9, whereby the surfaces of the nickel hydroxide particles are coated with specified proportions of .alpha.- and .beta.-forms of cobalt hydroxide (see Japanese Patent Public Disclosure No. 320733/1995);
4) by the same method as taught in Japanese Patent Public Disclosure No. 320733/1995, the surfaces of nickel hydroxide particles are coated with cobalt hydroxide to provide a bulk density which is at least 85% of the bulk density of the nickel hydroxide used as the base material (see Japanese Patent Public Disclosure No. 320736/1995); and
5) nickel hydroxide particles are coated with two layers of cobalt hydroxide, the inner layer being made of .alpha.- cobalt hydroxide and the outer layer of .beta.-cobalt hydroxide (see Japanese Patent Public Disclosure No. 320737/1995).
However, these methods have their problems. In the first method which solely relies upon the increased pH, a sufficiently uniform coating layer of cobalt hydroxide is not formed to permit satisfactory utilization of the nickel hydroxide particles. The second method is somewhat improved over the first method since it keeps the pH at a fixed value. But, on the other hand, the direct coating with a .beta.-form of cobalt hydroxide is not capable of forming a uniform coating layer of cobalt hydroxide. In addition, incorporating elemental Co or a Co compound is not a complete solution to the cracking problem. On the contrary, it adds an extra step and increases the process cost. The third method involves difficulty in controlling the proportions of the two different forms of cobalt hydroxide and, in addition, the use of the organic solvent adds to the cost of effluent treatment. In the fourth method, the bulk density of the coated nickel hydroxide particles is insufficient to prevent the decrease in positive capacity. In the fifth method, the coating layer of cobalt hydroxide is not sufficiently uniform to permit satisfactory utilization of the nickel hydroxide particles.
Thus, the cobalt hydroxide coated nickel hydroxide particles produced by the above-described conventional processes have not been completely satisfactory for use in the positive plate of alkali batteries.
An object, therefore of the present invention is to solve the aforementioned problems of the prior art by providing a high-density nickel hydroxide powder that comprises high-density nickel hydroxide particles coated with an .alpha.- or .beta.-cobalt hydroxide layer and which permits efficient utilization.